The need to store digital files, documents, pictures, images and other data continues to increase rapidly. In connection with the electronic storage of data, various systems have been devised for the rapid and secure storage of large amounts of data. Such systems may include a number of storage devices that are used in a coordinated fashion. In particular, data can be distributed across multiple storage devices such that data will not be irretrievably lost if one of the storage devices (or in some cases, more than one storage device) fails. An additional advantage that can be achieved by coordinating operation of a number of individual storage devices is improved data access and/or storage times. Examples of systems that can provide such advantages can be found in the various RAID (redundant array of independent disks) levels that have been developed.
Many data storage systems, such as RAID array systems, utilize one or more controllers. Such controllers typically provide a read cache and a write cache to improve the performance of the data storage system with respect to input/output (IO) operations. For example, in connection with write operations, a host can provide data for storage that is initially placed in the write cache of a data system controller. Once the data has been placed in the write cache, the controller indicates to the host that the write operation has been completed, even though the data has not yet been stored on a data storage device or devices. This frees the host to move on to other operations while the controller completes the relatively slow process of storing the data that has been placed in the write cache on the storage device or devices.
In many applications, the availability of data is a key concern. In particular, in many applications, users rely heavily on the data stored in the data storage system. In these types of applications, unavailability of data stored on the data storage system can result in significant loss of revenue and/or customer satisfaction. Employing a data storage system comprising a RAID array can enhance the availability of the stored data, since if a single disk drive fails, data may still be stored and retrieved from the system. In addition to the use of a RAID system, it is common to use redundant RAID controllers to further enhance the availability of a storage system. In such a situation, two or more controllers are used such that, if one of the controllers fails, the remaining controller will assume operations of the failed controller. The availability of the data storage system is therefore enhanced, because it can sustain a failure of a controller and continue to operate. When using dual controllers, each controller may conduct independent read and write operations simultaneously. This is known as an active-active configuration. In active-active configuration, write-back data and associated parity data are mirrored between the controllers.